I've been writing software in the last 10+ years, mainly as a consultant/freelancer. Within those years I got some exposure to the world of software developers and had the opportunity to work with more than a dozen different developer teams from 3 different country.

The answer to your question depends on what you mean by best developers, but in general you'll find most of the best developers already working for the best companies for developers.

Those companies already did the research, hired the best developers, gave them interesting and challenging problems to keep them motivated and offered compensation accordingly.

To find places with high density of the best developers find which office locations Google, Twitter, Amazon, etc. hires developers at.

Once you have a pool of potential candidates, put them to a test and get them write a small code/application for you. That way you'll learn about their actual coding skills and will also have a sense on their other skills like communication, problem solving, etc. It's a good idea to get an experienced developer to do a code review on the sample code the candidates wrote, to make sure the code is well organized, efficient and maintainable.

I hope I answered your question, if you have more, I'd be happy to help you with more details.

1. Where to fish: on average, where is the best developer for a given price ?
Generally where high quality education and low demand job market meet. Western Europe is perfect for this as it has some of the world's top schools while paying developers next to nothing (about half of what they cost in the US). Some countries in Asia come to mind as well although the cultural lack of creativity may be an issue. Don't be lured by the low cost countries, I've yet to see software developed in Eastern Europe or India that is better value for money than what is produced in the US or Western Europe.

2. What to catch: how can you tell whether they are the best software developers ?
It pretty much takes one to know one. Assuming you have a business mind you're pretty much lost without a tech advisor because you'll always pick the guy that makes the most sense to you without really knowing whether he's right or wrong.
Which is what I do too, with the major difference that I know how their mindset is going to affect practical outcomes.

Then come the practical questions:

1. So how do I pick a Tech Advisor ?
The short answer is pick me because I'm the best.
The long answer is that you need to find a way to filter profiles that match your needs. I could help you with that if you give me a call.

2. Any code, any language: this does not make sense: you will not find the best developers willingly using a technology that is not the best tool for any job. This is one of the easiest way to filter out bad candidates: ask them what technology they would use if they were given the choice and why. This also gives you the awesome opportunity to assess their self-improvement dynamics: do they have a good answer, are they willing to change their mind for a better one, etc.

I am not trying to sell you on calling me. Really, I am pretty busy with my businesses and consulting. However, I need more info before I could have a greater impact in helping you.

Ask, Ask, Ask, then Ask again.

Bonus:
Here is $10,000 worth of information for free and in a nutshell.

Concentrate on the 3 M's. There are actually 7, but 3 will do for now. These are Market, Message, and Media. They come in that order.

Who is your target market (customer, clients, buyers, users, etc.)?
Tailor your laser focused message for this target market.
What is the best media mix to get your message to that market?

Here's what you do...first, make it an offer that is so incredible that they cannot resist. Secondly, do all the work for them. Make it so easy to make the purchase now that they can do it virtually without effort. Thirdly, give them an incentive to act right now. Fourthly, offer an almost unbelievable guarantee. Fifth, offer a bonus for acting now. There are many other incredible steps, but these steps should help the novice to the professional sell anything.

Whether you are selling B2B or B2C, you have to focus on selling to only one person. You can actually sell to one person at a time while selling to millions at a time. They are one and the same. Don't get off track, what we call digital marketing selling is just selling in print. And that has not changed since Cluade Hopkins wrote "Scientific Advertising." Really long before he wrote the book.

The secret to success: I have had the pleasure of knowing and working with some of the biggest names in business, celebrities, actors, entrepreneurs, business people, and companies from startup to billion dollar operations. The number one reason for their success is doing what they know and love while doing it in new, creative, and innovative ways.

Ask, Ask, Ask. Have thick skin and learn from each "mistake." In a short while, the market will tell you what you need to do and who and what you need to ask. But get started now even if that just means asking a contact on LinkedIn.

While you are thinking, think big and think of something at least 1% better, newer, or different. And being cheaper is not a winning strategy.

Make decisions quickly and change decisions slowly..unless you are actually going off a cliff.

Remember these two 11 letter words...persistence and consistency. They are two of the most important tools ever invented.

Treat everybody you talk to and everybody you meet (including yourself) like each is your number one million dollar customer.

Bootstrap when possible and reasonable. Read "How To Get Rich" by Felix Dennis. Or better yet just remember the camel's nose in the tent story.

However, sometimes you just need to make a deal.

Listen, in any business you have to take some chances and some risks. Make sure you don't need a license and go for it. Remember, timid business people have skinny kids. Paraphrased from Zig Ziglar.